Adhesives used in manufacturing corrugated board are usually comprised of starch, a boron containing compound, caustic or another basic agent, and optionally a water resistance additive, where water resistance is needed, all provided in a water base. The major component of the adhesive is the starch which is gelatinized in the corrugating process after it penetrates the paper fiber. The other components are agents which modify the basic properties of the starch. For example, caustic usually in the form of sodium hydroxide, modifies the gelation temperature of the starch. Boron containing compounds are used to adjust or modify the adhesive tackiness, while also acting as buffering agents and to maintain the viscosity. Water resistance additives, which are optionally used, are typically derived from urea-formaldehyde, ketone formaldehyde or melamine-formaldehyde. Further, it is possible to use other poly-hydroxide compounds other than or in addition to starch in the adhesive compositions. For example, polyvinyl alcohol will work with the boron containing compounds in a manner similar to that of raw starch.
Starch based adhesives are typically made in a batch process by first preparing two separate portions, a cooked starch portion referred to as a "carrier" starch, and an uncooked portion referred to as "raw" starch. This is necessary to avoid creating an adhesive too viscous to be handled easily.
Typically, the carrier starch is prepared in a first tank by adding fresh water to starch, which may be either in raw or modified form. The mixture is agitated to form a starch slurry. Caustic is then admixed with the starch slurry and heated to initiate gelation and thereby produce the carrier starch paste portion.
In a second tank, water and starch, typically an unmodified raw starch, are agitated to form a raw starch slurry to which the boron containing compound, typically boric acid or borax is added. After the raw starch portion has been thoroughly mixed, the carrier starch portion is added to it and both are thoroughly mixed together to yield the corrugating adhesive, typically having a solids content of about 17 to 35% by weight and a viscosity in the range of about 150 to 650 centipoises.
The admixed starch adhesive is typically located in a tank of about 150 to 2,000 gallons capacity, with the mixture circulated to a corrugator on demand.
One problem with the prior batch systems is that adhesive preparation must be scheduled to satisfy demand as to timing, amount, specific formulation, viscosity, gel temperature and storage temperature. Generally, large storage tanks are necessary with their number and capacity depending on the variety of formulations and the number of use points within a facility.
After the adhesive has been formulated, any of a number of variables such as the degree of agitation in the tanks, the amount of circulation in the pipes, maintenance of particular temperatures, the continuing action of the basic agent (sodium hydroxide) on the starch in the paste and moisture loss, etc., may cause significant variations in the adhesive viscosity. Further, the costs for batch systems are high due to the need to have a number of storage tanks and specialized equipment for handling the viscous adhesive, while at the same time controlling the various parameters to avoid detrimental viscosity changes.
For example, once the admixture is created, the adhesive begins to age and it is subject to thickening or thinning and viscosity breakdown, which can detrimentally effect the properties of the corrugated product. While various methods have been attempted to control temperature so as to reduce the deterioration in properties with time, it is typical for there to be a significant amount of unused corrugating adhesive, which, once a run is complete, must be disposed of or worked into a new batch of adhesive. Because of the time required to use up a batch of adhesive, the corrugated product quality may vary due to the decrease in the beneficial properties of the corrugating adhesive with time.